Agreed. It's not like sysvinit ran on non-Linux systems either: they all have their own inits.
What *would* be nice is, when systemd calms down a bit and we start to see which of its features are used by other programs, if some other inits could start to gain those features. (I've never heard of another init-like program which could even be described as having features that other programs could use. systemd is a big step forward in that way.)